The ideal interconnect material for semiconductor circuitry will be inexpensive, easily patterned, have low resistivity, and high resistance to corrosion, electromigration, and "cold creep". Cold creep is a phenomena that results when a metal layer is deposited, at high temperature, on the surface of another material which has a smaller coefficient of expansion. When both materials are cooled, breaks may occur in the metal layer as the metal layer seeks to satisfy its own coefficient of expansion parameters. Such breaks may render a circuit non-conductive and, hence, useless.
Aluminum is most often used for interconnects in contemporary semiconductor fabrication processes primarily because it is inexpensive and relatively easy to etch. Because aluminum has poor electromigration characteristics and high susceptibility to cold creep, it is necessary to alloy aluminum with other metals.
As semiconductor device geometries shrink and clock speeds increase, it becomes increasingly desireable to reduce the resistance of the circuit metallization. The one criterium that is most seriously compromised by the use of aluminum for interconnects is that of conductivity. This is because the three metals with the lower resistivities--silver with a resistivity of 1.59 ohms/cm, copper with a resistivity of 1.73 ohms/cm, and gold with a resistivity of 2.44 ohms/cm--fall short in other important criteria. Silver, for example, is relatively expensive and corrodes easily, and gold is very costly and difficult to etch. Copper, with a resistivity nearly on par with silver, immunity from electromigration, high ductility (which provides high immunity to mechanical stresses generated by differential expansion rates of dissimilar materials in a semiconductor chip), high melting point (1083.degree. C. vs. 661.degree. C. for aluminum), fills most criteria admirably. However, copper is exceedingly difficult to etch in a semiconductor environment.
Contemporary aluminum metalization processes typically involve the blanket deposition of an aluminum metal layer, using either a sputtering operation or a low-pressure chemical vapor deposition (LPCVD) operation, followed by the etching of the metal layer to create the desired interconnect patterns for the circuitry. However, as semiconductor device dimensions have shrunk, it has become increasingly difficult to obtain adequate metalization step coverage within contact/via openings using sputtering deposition techniques. Although LPCVD provides much better step coverage than sputtering deposition for sub-micron devices, it suffers from several serious drawbacks, including inability to deposit aluminum that is doped with copper or other metals required for resistance to electromigration, cold creep, and silicon crystal formation in metal-to-silicon contact regions. In addition, metal deposition using either sputtering and LPCVD requires relatively high temperatures which promote impurity contamination and diffusion, in addition to creating coefficient of expansion incompatibilities between adjacent layers of dissimilar materials. Expansion incompatibilities may result in cold-creep-induced breaks in interconnect lines which may render the circuit useless. Furthermore, the etching of deposited metal layers to create interconnect lines is not an insignificant task. Aluminum and tungsten, for example, are relatively difficult to etch. This is especially true of aluminum alloys that exhibit minimal grain-boundary diffusion in the presence of an electric current (a necessary characteristic for resistance to electromigration). A defective metal layer etch may result in both shorted or open circuits. Sputter-deposition and LPCVD metalization methods are also adversely affected by particle contamination, which increases the probability of open circuits in the metal interconnect lines.
Given the problems associated with sputter and LPCVD metallizations, a number of electrodeposition processes have been developed for the metallization of semiconductor circuits. Most of these processes utilize gold as the principal metallization material, due to its ease of deposition and resistance to corrosion. However, since the conductivity of gold is little better than that of aluminum, gold metallization of semiconductor circuits is hardly the ideal solution. A typical gold circuit metallization process involves the steps of forming contact openings through a dielectric layer to expose an underlying metal layer or a conductively-doped silicon junction, sputter deposition of a barrier layer of a material such as titanium nitride or titanium-tungsten in combination with a superjacent palladium layer, creating a photoresist mask that exposes those areas of the circuit where metallization is desired (i.e. the regions that will become interconnect lines which, of course, incorporate the contact openings), electroplating a gold layer on top of the barrier layer, electroplating a thin rhodium layer on top of the gold layer, removing the photoresist mask, etching away the barrier layer and, finally, annealing the metallization pattern. A similar process even requires a sputter deposition of a thin gold layer prior to the electrodeposition of the majority of gold. Both processes are quite complex, and requires costly, rare metals.
At first glance, copper would appear to be an ideal metallization material for use in electrodeposition processes. However, electrodeposition processes for semiconductor circuitry utilizing copper as the principal metallization material have not been used, due to the difficulty of depositing copper metal on a barrier layer. Because copper, like most other metals, tends to diffuse into silicon junctions, altering the electrical characteristics thereof, the use of a barrier material such as titanium nitride, titanium-tungsten, or nitrided titanium-tungsten is essential.
Although there are a number of "textbook" copper electrodeposition baths, all are simply unusable in the context of semiconductor metallization in combination with conventional barrier materials. For example, a bath comprised of copper sulfate (CuSO.sub.4) and sulfuric acid (H.sub.2 SO.sub.4) produces poor adhesion of the deposited copper layer to the barrier material due to rapid oxide formation of the barrier material surface. In addition, the deposited copper layer tends to be of non-uniform thickness. When a bath comprised of copper pyrophosphate (Cu.sub.2 P.sub.2 O.sub.7), potassium pyrophosphate (K.sub.2 H.sub.2 P.sub.2 O.sub.7), ammonium hydroxide (NH.sub.4 OH) and ammonium nitrate (NH.sub.4 NO.sub.3) is used, metallic copper will not adhere to the barrier layer. The powdered copper that is deposited washes off with water. The moderately-hazardous bath comprised of copper fluoborate (Cu[BF.sub.4 ].sub.2), fluoboric acid (HBF.sub.4), and boric acid (HBO.sub.3) also produces only copper powder. The ultrahazardous bath comprised of copper cyanide (Cu[CN].sub.2), sodium cyanide (NaCN), and sodium hydroxide (NaOH) will produce an adherent copper metal layer on titanium-tungsten (though not on titanium nitride). However, this particular electrodeposition process will not satisfactorily fill contact openings. Another bath described in the literature, comprised of tetra-ammonium cuprite (Cu[NH.sub.3 ].sub.4, and ammonium hydroxide (NH.sub.4 OH), will not deposit copper on the barrier material.
What is needed is a relatively safe, simplified, copper electrodeposition process for the metallization of semiconductor circuitry which demonstrates excellent step coverage and adhesion characteristics.